The present invention relates to piercing a solid, cylindrical, metallic billet.
Piercing of such billets usually is carried out in a suitable piercing and press tool. In an initial phase, and as a preparatory step, an initial indent, perforation, blind bore or well is provided to be filled with lubricant for the piercing mandrel. Aside thereform, piercing of a billet is carried out by drilling a bore at first, and a conical mandrel widens the opening. Alternatively, a somewhat blunted piercing mandrel is being driven into the billet for one step piercing.
Predrilling followed by driving a concical mandrel into the drilled bore requires comparatively low press forces as to the mandrel operation, but the drilling is an additional step. This particular method is used today for piercing steel or steel alloy billets.
The use of a blunt mandrel refers to an older method but requires the use of a press exerting sufficiently large forces, that is very large, press forces upon the billet. Moreover, it was found that this particular method is not too reliable as far as centering of the pierced aperture or bore in relation to the outer periphery of the billet is concerned.